The present invention relates to improvements in a method of electric discharge machining and device thereof for conducting electric discharge machining on a workpiece by supplying electric power between an electrode and the workpiece.
FIG. 7 is a view showing an overall arrangement of a wire electric discharge machine which is an example of the conventional electric discharge device. In the drawing, reference numeral 2 is a workpiece, reference numeral 3 is a lower nozzle for spraying work fluid during working, reference numeral 4 is an NC device, reference numeral 5 is an upper nozzle for spraying work fluid during working, reference numeral 6 is a wire electrode for working, reference numeral 7 is a tapering device, in which U-axis drive unit and V-axis drive unit are housed, for moving the upper nozzle 5 with respect to the lower nozzle 3 in the case of tapering, reference numeral 8 is a surface plate table on which the workpiece 2 is set, reference numeral 9 is a power source for supplying electric power for working to the workpiece 2 and the wire electrode 6, reference numeral 21 is a wire bobbin round which the wire electrode 6 is wound, reference numeral 22 is a pulley for changing a direction of the wire electrode 6, reference numeral 23 is a tensioner for giving a constant intensity of tension to the wire electrode 6, reference numeral 24 is a wire recovery roller for feeding the wire electrode 6, reference numeral 27 is an X-axis servo motor for moving the surface plate table 8 in the direction of X-axis, reference numeral 28 is a Y-axis servo motor for moving the surface plate table 8 in the direction of Y-axis, reference numeral 29 is a servo motor of Z-axis for moving the tapering device 7 and the upper nozzle 5 in the direction of Z-axis, reference numeral 30 is a servo motor of U-axis for moving the tapering device 7 in the direction of U-axis, and reference numeral 31 is a servo motor of V-axis for moving the tapering device 7 in the direction of V-axis. In this case, electric power supply to the workpiece 2 and the wire electrode 6 is omitted here.
FIGS. 8a and 8b are arrangement views of a surface plate table of a conventional wire electric discharge machine for obtaining smoother surface roughness on a work surface. In the drawing, reference numeral 1 is an insulating member, reference numeral 2 is a workpiece, reference numeral 3 is a lower nozzle for spraying work fluid in the process of working, reference numeral 5 is an upper nozzle for spraying work fluid in the process of working, reference numeral 10 is a feeder cable, reference numeral 11 is a finish feeder cable, reference numeral 12 is a contactor for opening and closing the feeder cable 10 and the finish feeder cable 11, reference numeral 13 is an auxiliary contactor for opening and closing electric power supply from the feeder cable 10 to the workpiece 2, and reference numeral 14 is a work tank for storing work fluid so that the workpiece 2, lower nozzle 3 and upper nozzle 5 can be dipped in the work fluid.
In FIGS 8a and 8b, electric power supply to the wire electrode is not shown, for clarity.
Next, a method of electric power supply will be explained below. For example, when working is conducted under the condition that the surface roughness of a work surface is more than 3 xcexcmRmax, both the feeder cable 10 and the finish feeder cable 11 are used for working, and when working is conducted under the condition that the surface roughness of a work surface is not more than 3 xcexcmRmax, which is smoother than the above surface roughness, only the finish feeder cable 11 is used for working so that an intensity of electric discharge energy can be reduced to conduct working with high accuracy. The above is an example in which material of the workpiece 2 is SKD11, thickness of the workpiece is 20 mm, material of the wire electrode 6 is brass and diameter of the wire electrode 6 is 0.2 mm. In the case where material of the workpiece 2 is changed or thickness of the workpiece 2 is changed, and in the case where material of the wire electrode 6 is changed or diameter of the wire electrode 6 is changed, surface roughness of the work face, by which an intensity of electric power to be supplied is changed, is changed. In the above example, surface roughness 3 xcexcmRmax is changed.
A case in which surface roughness of the work face, by which an intensity of electric power to be supplied is changed, is 3 xcexcmRmax will be explained as follows.
In the structure shown in FIG. 8(a), the insulating member 1 is arranged in an upper portion of the surface plate table 8, and the workpiece 2 is set in an upper portion of the insulating member 1. The frequency of working determined by the required surface roughness of a product is inputted into the working program, and the energy setting of the working electric power source 9 with respect to the frequency of working is also inputted into the working program, and then the program is carried out.
In the case of working in which surface roughness of the work face is not more than 3 xcexcmRmax, electric power supply from the feeder cable 10 is stopped by the contactor 12, and electric power is supplied only by the finish feeder cable 11. When the auxiliary contactor 13 is opened at the same time, the workpiece 2 is insulated except for the supply of electric power by the finish feeder cable 11. Therefore, no electric current is supplied to the workpiece 2 except for the electric current supplied by the finish feeder cable 11. Therefore, it becomes possible to conduct working of smoother surface roughness.
However, the structure shown in FIG. 8(a) has the following disadvantages. The insulating member 1 is arranged in an upper portion of the surface plane table 8. Therefore, it is necessary to attach the insulating member 1 onto the surface plane table 8 at a site where working is actually conducted. Further, it is also necessary to fix the workpiece 2 to the insulating member 1. For the above reasons, compared with a case in which the workpiece 2 is directly attached onto the surface plane table 8, it takes much longer time for preparation. As a result, the manufacturing cost of the parts manufactured by electric discharge might be increased.
In the structure shown in FIG. 8(b), the workpiece 2 is directly arranged in an upper portion of the surface plane table 8. According to the structure shown in FIG. 8(b), the frequency of working determined by the required surface roughness of a product is inputted into the working program, and the energy setting of the working electric power source 9 with respect to the frequency of working is also inputted into the working program, and then the program is carried out. In the case of working in which surface roughness of the work face is not more than 3 xcexcmRmax, electric power supply from the feeder cable 10 is stopped by the contactor 12, so that electric power is supplied only by the finish feeder cable 11. Therefore, no electric current is supplied to the workpiece 2 except for the electric current supplied by the finish feeder cable 11. Therefore, it becomes possible to conduct working of smoother surface roughness.
However, the structure shown in FIG. 8(b) has the following disadvantages. In the structure shown in FIG. 8(b), the insulating member 1 is attached to a lower portion of the surface plate table 8. However, when the working tank 14 and the surface plane table 8 are arranged close to each other and an area in which the working tank 14 and the surface plane table 8 are opposed to each other is large, the working tank 14 and the surface plane table 8 compose a type of condenser. Therefore, when AC voltage is impressed between the working tank 14 and the surface plane table 8, an electric current flows between them although they are electrically insulated from each other. As a result, an intensity of electric power required for working is increased, and it becomes impossible to obtain a predetermined surface roughness on the work surface. In the case where a larger insulating member is used so that a distance between the working tank 14 and the surface plane table 8 can not be too short, the material cost and the manufacturing cost of the insulating member made of expensive material such as ceramics are increased.
In order to meet the demand of high accuracy and short time for delivery which is requested by the present market in which parts worked by the electric discharge machine are traded, it is necessary that high working accuracy and high working speed are compatible with each other.
The conventional methods shown in FIGS. 8(a) and 8(b) for obtaining smoother surface roughness have the following disadvantages. Although the surface roughness on a work face becomes smoother, since the insulating member 1 is used in these method, electric power is supplied by the feeder cable and the finish feeder cable. Accordingly, by the influence of inductance of the feeder cable and the finish feeder cable, the discharge peak current is decreased, especially, the working speed in the case of rough working process is decreased. Therefore, it is impossible that the working accuracy and the working speed are compatible with each other although it is strongly requested by the market in which parts manufactured by the electric discharge machines are traded.
The present invention has been accomplished to solve the above conventional problems. It is an object of the present invention to provide a method of electric discharge machining and device thereof in which an enhancement in the working accuracy and an increase in the working speed can be made compatible with each other and further the manufacturing cost of parts manufactured by the electric discharge machine can be reduced.
The first invention provides a method of electric discharge machining characterized in that: when the surface roughness on a work face of the workpiece is more than a predetermined value, a bladder capable of expanding or contracting by the pressure of fluid filled in the bladder is contracted, and the work fluid is filled in a space between the working tank and the surface plate table and working is conducted on the workpiece; and when the surface roughness on a work face of the workpiece is not more than a predetermined value, the bladder is expanded, and a quantity of work fluid in the space between the work tank and the surface plate table is reduced and working is conducted on the workpiece.
The second invention provides a method of electric discharge machining according to the first invention, characterized in that: when the surface roughness on a work face of the workpiece is more than a predetermined value, working is conducted while electric power supply is increased by an electric power supply controlling means for controlling work electric power; and when the surface roughness on a work face of the workpiece is not more than a predetermined value, working is conducted while electric power supply is decreased by the electric power supply controlling means.
The third invention provides a method of electric discharge machining characterized in that: when the surface roughness on a work face of the workpiece is more than a predetermined value, working is conducted while substance having a high dielectric constant is filled in a shielding space formed between the surface plate table and the work tank with respect to the work fluid in the work tank; and when the surface roughness on a work face of the workpiece is not more than a predetermined value, working is conducted while substance having a low dielectric constant is filled in the shielding space.
The fourth invention provides a method of electric discharge machining according to the third invention, characterized in that: when the surface roughness on a work face of the workpiece is more than a predetermined value, working is conducted while electric power supply is increased by an electric power supply controlling means for controlling work electric power; and when the surface roughness on a work face of the workpiece is not more than a predetermined value, working is conducted while electric power supply is decreased by the electric power supply controlling means.
The fifth invention provides an electric discharge machine comprising: an insulating member arranged between the surface plate table and the work tank, connecting the surface plate table with the work tank, supporting the surface plane table; a bladder arranged between the surface plate table and the work tank, expanded and contracted by the pressure in the bladder; and an expanding and contracting device capable of expanding and contracting the bladder by changing the pressure in the bladder, and when the surface roughness on a work face of the workpiece is more than a predetermined value, a bladder capable of expanding or contracting by the pressure of fluid filled in the bladder is contracted, and the work fluid is filled in a space between the working tank and the surface plate table and working is conducted on the workpiece; and when the surface roughness on a work face of the workpiece is not more than a predetermined value, the bladder is expanded, and a quantity of work fluid in the space between the work tank and the surface plate table is reduced and working is conducted on the workpiece.
The sixth invention provides an electric discharge machine according to the fifth invention, further comprising an electric power supply controlling means for controlling work electric power, electric power supply is increased by the electric power supply controlling means when the surface roughness of a work face of the workpiece is more than a predetermined value, and electric power supply is decreased by the electric power supply controlling means when the surface roughness of a work face of the workpiece is not more than a predetermined value.
The seventh invention provides an electric discharge machine comprising: an insulating member arranged between the surface plate table and the work tank, connecting the surface plate table with the work tank, supporting the surface plane table; a shielding space formed between the surface plane table and the work tank, with respect to the work fluid in the work tank; and a fluid supplying and recovering device having a function supplying substance having a high dielectric constant to the shielding space and recovering substance having a high dielectric constant from the shielding space, and when the surface roughness on a work face of the workpiece is more than a predetermined value, working is conducted while substance having a high dielectric constant is filled in the shielding space by the fluid supplying and recovering device, and when the surface roughness on a work face of the workpiece is not more than a predetermined value, working is conducted while substance having a low dielectric constant is filled in the shielding space by the fluid supplying and recovering device.
The eighth invention provides an electric discharge machine according to the seventh, further comprising an electric power supply controlling means for controlling work electric power, electric power supply is increased by the electric power supply controlling means when the surface roughness of a work face of the workpiece is more than a predetermined value, and electric power supply is decreased by the electric power supply controlling means when the surface roughness of a work face of the workpiece is not more than a predetermined value.
Since the present invention is composed as described above, the following effects can be provided.
According to the method of electric discharge machining of the first and the third invention, when the surface roughness of a work face of the workpiece is more than a predetermined value, it is possible to replenish an intensity of electric power supply to the workpiece. Therefore, it becomes possible to prevent the working speed from decreasing. When the surface roughness of a work face of the workpiece is not more than a predetermined value, insulation of the surface plane table can be ensured. Therefore, it is possible to obtain a smoother work surface. Accordingly, an enhancement of the work accuracy and an increase in the work speed can be made compatible with each other. Further, it is possible to reduce a size of the insulating member to be used. Therefore, it becomes unnecessary to use expensive materials for the insulating member, and the manufacturing cost can be greatly reduced.
The methods of electric discharge machining according to the second and the fourth invention can provide the same effects as those of the first and the third invention, and electric power supply is changed over according to the surface roughness of a predetermined value of a work face of the workpiece. Therefore, the work accuracy and the work speed can be further enhanced.
The electric discharge machines according to the fifth and the seventh invention can provide the same effects as those of the first and the third invention.
The electric discharge machines according to the sixth and the eighth invention can provide the same effects as those of the second and the fourth invention.